THE TIGA GENE IS A TRANSCRIPTIONAL FUSION OF GLYCOLYTIC GENES ENCODING TRIOSE-PHOSPHATE ISOMERASE AND GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE IN OOMYCOTA

Genes encoding triose-phosphate isomerase (TPI) and glyceraldehyde-3-p hosphate dehydrogenase (GAPDH) are fused and form a single transcripti onal unit (tigA) in Phytophthora species, members of the order Pythial es in the phylum Oomycota. This is the first demonstration of glycolyt ic gene fusion in eukaryotes and the first case of a TPI-GAPDH fusion in any organism. The tigA gene from Phytophthora infestans has a typic al Oomycota transcriptional start point consensus sequence and, in com mon with most Phytophthora genes, has no introns. Furthermore, Souther n and PCR analyses suggest that the same organization exists in other closely related genera, such as Pythium, from the same order (Oomycota ), as well as more distantly related genera, Saprolegnia and Achlya, i n the order Saprolegniales. Evidence is provided that in P. infestans, there is at least one other discrete copy of a GAPDH-encoding gene bu t not of a TPI-encoding gene. Finally, a phylogenetic analysis of TPI does not place Phytophthora within the assemblage of crown eukaryotes and suggests TPI may not be particularly useful for resolving relation ships among major eukaryotic groups.